Filter media for active field polarized media air cleaner

ABSTRACT

Filter media for an active field polarized media air cleaner includes two layers of dielectric material with a higher resistance air permeable screen sandwiched between the lower resistivity electric layers. The filter media may further include a mixed fiber filter layer having fibers from different sides of the triboelectric scale. The filter media may further include a layer of relatively higher density dielectric material followed by a layer of relatively lower density dielectric material.

FIELD OF THE INVENTION

The present invention relates generally to air cleaning systems and isparticularly directed to filter media for air cleaners of the type thatuse an electrostatic field to polarize a media and to polarize particlesto increase the particle collection efficiency on the media.

BACKGROUND OF THE INVENTION

The principal of electrostatic attraction has been used for many yearsto enhance the removal of contaminants from air streams. There are threeprimary categories of air electrostatic cleaners: electrostaticprecipitators, passive electrostatic filters and active field polarizedmedia air cleaners, which are sometimes known under different terms.

Electrostatic precipitators charge particles and then capture them onoppositely charged and/or grounded collection plates.

A passive electrostatic filter (also know as an electret) employs amedia (or combination of different media) that through some combinationof treatment and/or inherent properties has an electrostatic charge.Particles entering the filter media that have an electrostatic chargeare attracted to the charged media filter materials that have theopposite electrostatic charge.

An active field polarized media air cleaner uses an electrostatic fieldcreated by a voltage differential between two electrodes. A dielectricfilter media is placed in the electrostatic field between the twoelectrodes. The electrostatic field polarizes both the media fibers andthe particles that enter, thereby increasing the efficiency of the mediaand the air cleaner. A dielectric material is an electrical insulator ora substance that is highly resistant to electric current that can alsostore electrical energy. A dielectric material tends to concentrate anapplied electric field within itself and is thus an efficient supporterof electrostatic fields.

A further electrostatic air filter design is disclosed in CanadianPatent No. 1,272,453, in which a disposable rectangular cartridge isconnected to a high voltage power supply. The cartridge consists of aconductive inner center screen, which is sandwiched between two layersof a dielectric fibrous material (either plastic or glass). The twodielectric layers are, in turn, further sandwiched between two outerscreens of conductive material. The conductive inner center screen israised to a high voltage, thereby creating an electrostatic fieldbetween the inner center screen and the two conductive outer screensthat are kept at an opposite or ground potential. The high voltageelectrostatic field polarizes the fibers of the two dielectric layers.

The air cleaners may be installed in a variety of configurations andsituations, both as part of a heating ventilating and air conditioning(HVAC) system and in standalone air moving/cleaning systems. In smallerHVAC systems (e.g. residential and light commercial), the air cleanerpanels are often installed in a flat configuration (perpendicular to theairflow) or in angled filter tracks. In larger systems, banks of airfilters are typically arranged in a V-bank configuration where multipleseparate filters are positioned to form a Z-fold filter perpendicular tothe axis of airflow.

SUMMARY OF THE INVENTION

The invention is embodied in several individual improvements to filtermedia for active field polarized media air cleaners and combinationsthereof. The individual features of the present invention are asfollows:

1. In particular, the filter media of the present invention includes twolayers of fibrous dielectric material (such as polyester) with a higherresistance air permeable material (such as a fiberglass screen)sandwiched between the lower resistance dielectric (polyester) layers.

2. In another embodiment of the present invention, the filter mediaincludes a layer of fibrous dielectric material forming a mixed fiberlayer having fibers from different ends of the triboelectric series ofmaterials (triboelectric scale) for use in an active field polarizedmedia air cleaner.

3. In yet another embodiment of the present invention, the filter mediaincludes a layer of relatively higher density dielectric material (suchas fibrous polyester), followed by a layer of relatively lower densitymaterial (such as less dense fibrous polyester).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of filter media incorporating ahigher resistance air permeable screen between fibrous pads ofdielectric material having a relatively lower resistance in accordancewith the present invention.

FIG. 2 is a cross-sectional diagram of filter media incorporating afibrous pad of dielectric material and a mixed fiber layer having fibersfrom different ends of the triboelectric scale in accordance with thepresent invention.

FIG. 3 is a cross-sectional diagram of filter media incorporatingfeatures from FIGS. 1 and 2 in accordance with the present invention.

FIG. 4 is a cross-sectional diagram of filter media incorporating alayer of higher density dielectric material followed by a lower densitydielectric material in accordance with the present invention.

FIG. 5 is a cross-sectional diagram of filter media incorporatingfeatures from FIGS. 1 through 4 in accordance with the presentinvention.

FIG. 6 is a cross-sectional diagram of filter media incorporatingfeatures from FIGS. 1 and 5 in accordance with the present invention.

FIG. 7 is a cross-sectional diagram of a filter media incorporatingfeatures from FIGS. 3 and 6 in accordance with the present invention.

FIG. 8 is a cross-sectional diagram of filter media incorporating amixed fiber layer having fibers from different ends of the triboelectricscale in accordance with the present invention.

FIG. 9 shows a photocatalytic device embodiment.

FIG. 10, shows an alternative photocatalytic device embodiment.

DETAIL DESCRIPTION

An embodiment of an active field polarized media air cleaner inaccordance with the present invention is shown in FIG. 1. In FIG. 1 (asin FIGS. 2-7) airflow through the filter is downward from the top of thediagram to the bottom of the diagram. The filter consists of a framethat holds the filter media.

In one embodiment of the invention, the filter media itself consists ofa dielectric media support frame 120, a first pad of fibrous dielectricmaterial 16A, fiberglass mesh screen 14A, a second pad of fibrousdielectric material 16B, a center screen 13, a third pad of fibrousdielectric material 16C, another fiberglass mesh screen 14B and a fourthpad of dielectric filter material 16D. The filter frame that holds thefilter media consists of a first conductive holding frame 116A with afirst conductive outer screen 12A, and a second conductive holding frame116B with a second conductive outer screen 12B. While for the sake ofclarity, the same basic configuration using the media frame 120 andholding frames 116A,B, are used for drawings, these are but one possibleembodiment. The essential elements of the current invention are thevarious configurations of media between two electrodes in a polarizedmedia air cleaner. While there are certain advantages to the specificembodiments disclosed in the illustrations, the center screen need notgo all the way to the edge, nor have a frame around-it and the medialayers.

The first pad of dielectric filter material 16A is attached to thedielectric media support frame 120 and/or the center electrode 13 by asuitable means such as adhesive material 121A or ultrasonic welding. Thefourth pad of dielectric filter material 16D is attached to thedielectric media support frame 120 by a suitable means, such as adhesivematerial 121B or ultrasonic welding. In embodiments with no mediasupport frame, the various layers of media 16A-D and 14A and B andcenter screen 13 would be typically be attached together by a suitablemeans such as adhesives, ultrasonic welding, sewing or clamping. Thefirst conductive outer screen 12A is held in place by a first conductiveholding frame 116A. The second conductive outer screen 1213 is held inplace by a second conductive holding frame 116B.

In operation, one terminal of a high voltage power supply 108 isconnected to center screen 13. The other terminal of the high-voltagepower supply 108 is coupled to the first conductive outer screen 12A andthe second conductive outer screen 12B, which is held typically atground potential.

Particles in the incoming air passing through dielectric filter material16A, 16B, 16C and 16D of the active field polarized media air cleaner ofFIG. 1 are polarized by the electric field therein and collected on thefirst and second pads of dielectric filter material 16A, 16B, 16C and16D.

Filter media of the present invention include two layers of fibrousdielectric material with a higher resistance air permeable materialsandwiched between the lower resistance dielectric layers. While othermaterial combinations are possible, specifically, in FIG. 1 a fiberglassscreen 14A is sandwiched between polyester layer 16A and polyester layer16B, which is disposed above the center screen 13. Similarly, below thecenter screen 13 is a fiberglass screen 14B sandwiched between polyesterlayer 16C and polyester layer 16D.

It has been found that this disposition of materials allows for a higherand more stable voltage differential between the electrodes. Thisincreases particle removal efficiencies as higher voltage means higherfield strength and therefore higher efficiencies. Specifically, it hasbeen found the above disposition of materials allows for up to a 25%higher voltage without arcing and spraying between electrodes.

Filter media of the present invention include a mixed fiber layer in anactive field polarized media air cleaner, said mixed fiber layer havingfibers from different parts of the triboelectric series of materials.Most materials will generate and store some static electricity. Thecapacity of the material to generate and store an electric chargedefines where it belongs on the triboelectric scale.

Triboelectric Series of Materials Causing Static Electricity

Some materials create more static electricity than others. Since staticelectricity is the collection of electrically charged particles on thesurface of a material, various materials have a tendency of eithergiving up electrons and becoming positive (+) in charge, or attractingelectrons and becoming negative (−) in charge. The triboelectric seriesis a list of materials, showing which have a greater tendency to becomepositive (+) and which have a greater tendency to become negative (−).The list is used to determine which combinations of materials create themost static electricity.

For illustration purposes, some common materials are listed belowaccording how well they create static electricity when rubbed withanother material, as well as what charge the material will possess. Thelist is not an exhaustive list and every material fits somewhere is thepositive or negative triboelectric scale.

Materials that Become Positive in Charge

The following materials will tend to give up electrons when brought incontact with other materials. They are listed from those with thegreatest tendency to give electrons to those that barely give upelectrons.

Comments

Dry human skin Greatest tendency to giving up electrons and becominghighly positive (+) in charge Leather Rabbit fur Fur is often used tocreate static electricity Glass The glass on your TV screen gets chargedand collects dust Nylon Wool Lead A surprise that lead would collect asmuch static electricity as cat fur Cat fur Silk Aluminum Gives up someelectrons Paper

Materials that are Neutral

There are very few materials that do not tend to readily attract or giveup electrons when brought in contact or rubbed with other materials.

Comments

Cotton Best for non-static clothes Steel Not useful for staticelectricity

Materials that Become Negative in Charge

The following materials will tend to attract electrons when brought incontact with other materials. They are listed from those with the leasttendency to attract electrons to those that readily attract electrons.

Comments

Wood Attracts some electrons, but is almost neutral Amber Hard rubberSome combs are made of hard rubber Nickel, Copper Copper brushes used inWimshurst electrostatic generator Brass, Silver Gold, Platinum It issurprising that these metals attract electrons almost as much aspolyester Polyester Clothes have static cling Styrene Packing materialseems to stick to everything (Styrofoam) Saran Wrap You can se how SaranWrap will stick to things Polyurethane Polyethylene (like Scotch PullScotch Tape off surface and it will Tape) become charged PolypropyleneVinyl (PVC) Many electrons will collect on PVC surface Silicon TeflonGreatest tendency of gathering electrons on its surface and becominghighly negative (−) in charge

The best combinations of materials to create static electricity would beone from the positive charge list and one from the negative charge list.However, a moderate amount of static electricity can be created from twomaterials on the positive charge list or two materials on the negativecharge list. For example, when two materials that tend to give upelectrons are rubbed together, the one with the greatest tendency willmoderately become positive (+) in charge Likewise, when two materialsthat tend to attract electrons are rubbed together, the one with thegreatest tendency will moderately become negative (−) in charge.

A filter media of the present invention for use in an active fieldpolarized media air cleaner using a mix of fibers from different partsand/or preferably different sides of the triboelectric scale is shown inFIG. 2. Specifically, filter layer 15A contains a mix of fibers fromdifferent sides of the triboelectric scale (mixed triboelectric filterlayer). The different fibers of filter layer 15A may be interwoven andmixed together throughout filter layer 15A, or in the alternative, thedifferent fibers of filter layer 15A may be first and second separatesheets of filter material placed in contact with each other. That is, afirst sheet of filter material is made of fibers from one side of thetriboelectric scale and a second sheet of filter material is made offibers from the other side of the triboelectric scale. The first andsecond sheets of filter material are placed in contact with each otherto form the mixed triboelectric filter layer' 15A.

Mixed triboelectric filter layer 15B is similar to mixed triboelectricfilter layer 15A. The important feature of mixing together (byinterweaving or bringing into contact) fibers from different sides ofthe triboelectric scale is that the mixture of such fibers produces sitsof relative positive and negative charge on the fibers within such mixedtriboelectric fiber layers 15A, 15B. Such integrated materials areavailable are available from, among others, Alhstrom Air Media who's HPseries of material is a mix of modacrylic and polypropylene andHollingsworth and Vose who's Technostat materials are a mix of acrylicand polypropylene.

It is well known in the manufacture and design of passive electrostaticfilters that the proper mix of materials from different sides of thetriboelectric scale will boost the efficiency of the media beyond whatwould be anticipated solely from the density of the media, i.e. from thepassive mechanisms of the media. Other types of passive electrostaticfilters have charges imposed on the media by a variety of techniques.One issue with passive electrostatic filters is that the initialefficiencies due to electrostatic attraction actually decline as thefibers become covered with contaminants and/or gradually discharge dueto a variety of factors (humidity, chemicals, and temperature).

While putting many filter medias in an electrostatic field can increasetheir efficiency, this is not universally the case. In fact, manypassive electrostatic media show no improvement or actually performworse. However, it has been found that putting triboelectric typeelectret media in a polarizing field does improve its effectiveness andeliminate the efficiency dip that is seen. The triboelectric layer tendsto be relatively thin, it may therefore be used in one or more layers,together or separate, at various positions within the air cleaner media,i.e. with other media materials positioned on either or both sides ofit.

In another embodiment of the present invention, above the mixedtriboelectric filter layer 15A is a relatively sparse fibrous layer 16E.The filter media structure above the center screen 13 is repeated belowthe center screen 13, namely a second relatively sparse fibrous layer16F above a second mixed triboelectric filter layer 15B. The relativelysparse layers could be a variety of materials or different materialsfrom each other.

A filter media of the present invention for use in an active fieldpolarized media air cleaner which combines both a fiberglass centerscreen 14A, 14B and a mix of fibers from different sides of thetriboelectric scale is shown in FIG. 3. The filter media in FIG. 3 is acombination of the filter media shown in FIGS. 1 and 2.

This combination combines the benefits of each embodiment, allowing forthe for maximum system efficiency.

A filter media incorporating a layer of higher density dielectricmaterial followed by layer of low density dielectric material is shownin FIG. 4. The filter media shown in FIG. 4 is similar to that shown inFIG. 2. However in FIG. 4, an additional filter layer 25A of relativelylower density material is disposed after filter layer 16E, which is ofrelatively higher density material.

Another filter media incorporating a layer of lower density dielectricmaterial following a layer of higher density dielectric material isshown in FIG. 5. The filter media shown in FIG. 5 is similar to thatshown in FIG. 3. However, in FIG. 5 an additional filter layer 25A ofrelatively less dense material is positioned after filter layer 16B.Additionally, in FIG. 5 a second triboelectric filter layer 25B ofrelatively less dense material is placed after filter layer 16D at theend of the airflow through the active field polarized media air cleaner.

The benefit of these embodiments is a reduction in resistance toairflow. The densest layer of media will have the highest resistance toairflow. If the densest layer is against one of the electrodes, its areawill be effectively reduced by that of the electrode. This wouldincrease the air velocity through the remaining area and increase theresistance to airflow. By putting a less dense layer between theelectrode and the densest layer, it increases the air speed through theless dense material instead of the denser material thereby reducingresistance to airflow significantly.

In FIG. 6, the portion of the filter media above the center screen 13 isthe same as that shown in FIG. 1; the portion of the filter media belowthe center screen 13 is the same as that shown in FIG. 5.

This embodiment provides superior loading characteristics. By capturingthe larger particles or particles of lower density and/or lowermomentum, on the less dense upstream layers, the more dense layers donot become clogged and are able to collect primarily the smaller (i.e.,higher density and/or higher momentum) particles and therefore have alonger service life. The media thus allows a homogenous distribution ofparticulate through the volume of the media.

In FIG. 7, the portion of the filter media above the center screen 13 isthe same as that shown in FIG. 3; the portion of the filter media belowthe center screen 13 is the same as that shown in FIG. 5 or 6.

In FIG. 8, the filter media 15C, 15D above the center screen 13 is amixed fiber layer having fibers from different ends of the triboelectricscale in accordance with the present invention.

In a further embodiment of the invention, one of the outer most layersof media could be treated with a photocatalytic material. The aircleaner could then be coupled with a UV light for the breakdown of gasphase contaminants. Hydroxyls produced in this embodiment couldinactivate biologicals and breakdown gas phase contaminants. In such anembodiment, under the influence of UV light, the media creates hydroxylradicals and super-oxide ions to react with the captured and airbornebioaerosols and gas phase contaminants. The photocatalytic layer couldbe the furthest downstream layer. This would keep it substantially freeof particle contamination.

In a further embodiment of the invention, the external screen/electrodeof the filter frame is treated with the photo catalyst.

In a further embodiment of the invention the center screen would haveodor absorbing properties, such as a carbon impregnated foam or mesh.

The downstream layer of lower density material could be treated with acatalyst for breaking down VOC's, other reactive gas phase contaminantsand/or Ozone and/or biological contaminants.

At least one of the external screens and/or a layer of media can betreated with a photocatalyst that tends to destroy gaseous impuritiessuch as VOC's and biological contaminants in the presence of light,typically UV light. The latter configuration could be coupled with oneor more UV sources in close proximity to thereby bring about thephotocatalytic effect. The resulting integration of components wouldsignificantly reduce the cost of applying photocatalysis to the airstream. The catalyst may be applied to either the upstream or thedownstream screen. The system may include central UV lights to shine ontreated screens both upstream and downstream of the treated screens. Inthe case of applying the photocatalyst to a layer of media, thepreferred embodiment would be to have it be the most downstream layer,as this layer would be less likely to be fouled by contaminants.

FIG. 9 shows a photocatalytic device embodiment. Phtocatalytic Oxidation(PCO) has been used to purify various liquid and gas streams but in PCO,there is sometimes an issues where the catalyst may not adhere to thesubstrate onto which it has been applied and PCO process efficiency.Titanium dioxide may be used as the photocatalyst that coats the filteror electrodes so that PCO may be used to purify the stream.

As shown in FIG. 9. placing the substrate within an electrostatic fieldresults in both better adherence of the catalyst to the substrate 901and a quicker reaction time. For example, as shown in FIG. 9, if thecatalyst is applied to a non-woven fiberglass material 901 and thatmaterial is placed between two electrodes 912A, 912B with a voltagedifferential of 5-15 kv dc and spaced approximately 0.5″ sapart, under aUV light source 940, the catalyst is held onto the fiberglass material901 by the resultant polarizing field and shed less readily. Further,the energy of the field itself speeds the breakdown of volatilecompounds in the airstream.

In preliminary testing, TVOC levels were brought down almost twice asquickly with the same catalyst when it was placed in an electrostaticfield. The experiment was as follows: A u-shaped duct with fan wereconfigured to draw and return air from and to an approximately 1,000cubic foot room. Tolulene was poured on a cloth and the cloth left inthe space for several minutes. An Aircuity Optima unit was used tomeasure TVOC levels in the room. A V-bank air cleaner was placed in theduct with a 254 urn UVC lamp upstream. A circulating fan was set to1,000 cfm. The air cleaner frames could hold a media and were able tocreate and electrostatic field inside the media. After 25 minutes, withno filter, TVOC levels dropped 6%. With UV lamp on and a catalyst coatedmedia with no electrostatic field, there was a 12% drop. With the samearrangement and the electrostatic field energized, there was a 24% drop.

Such a device could be applied to a variety of HVAC and air movingsystems for purification of the airstream. The catalyst could also beapplied to a woven material and could be a variety of materials otherthan glass. The catalyst coated material could be sprayed or dipped inthe catalyst.

In another embodiment shown in FIG. 10, the catalyst coated material1012 could be one layer of a multiple layer filter media containing boththe coated material 1012 and a non-coated layer 1020 between electrodes1012A, 1012B in the presence of a UV light source 1040 s.

The electrodes could be made from a variety of conductive materials andmay be electrically uniform and include evenly spaced grids on eitherside of the coated media material. Further they may be as open aspossible to allow the maximum light penetration into the coatedmaterial. The electrodes themselves could be coated with the catalyst aswell.

In all cases, the device may be put in close proximity to a UV lightsource(s) to supply primary energy for the catalytic reaction. Hydroxylsproduced in this scenario could inactivate biologicals and breakdown gasphase contaminants. In such an embodiment, under the influence of UVlight, the media creates hydroxyl radicals and super-oxide ions to reactwith the captured and airborne bioaerosols and gas phase contaminants.

While the inventions described above have made reference to variousembodiments, modifications can be made to the structure and elements ofthe invention without departing from the spirit and scope of theinvention as a whole. In particular, various layers or elements could becombined or repeated to achieve various effects. For example, while onefigure shows the basic concept of the air cleaner, another figure showsthe configuration of one type of assembled system. While for the sake ofclarity, the various elements have been shown as separate layers, two ormore of the “layers” may be combined into a single layer or material.

The invention(s) disclosed above could be used in variety of ways,including, but not limited to, use in HVAC systems, self-containedfilter/fan units, and industrial air cleaning systems, and dustcollectors. While the above embodiments primarily describe flat filterconfigurations, the inventions could be adapted to other configurationsas well, including but not limited to V-bank groupings of multiple flatpanels, interconnected groupings of panel and V-Bank units andcylindrical filters for dust collection systems.

What is claimed is:
 1. An active field polarized media air cleanercomprising: a first conductive outer screen; a second conductive screensubstantially parallel to said first conductive outer screen; a pad ofmaterial, the pad of material being disposed between said firstconductive outer screen and said second conductive screen, wherein thematerial comprises multiple layers and at least one of the multiplelavers is a different material from another of the multiple layers; anda high-voltage power supply having first and second terminals, the firstterminal of said high voltage power supply being connected to saidsecond conductive screen, said second terminal of said high-voltagepower supply being coupled to said first conductive outer screen.
 2. Anactive field polarized media air cleaner in accordance with claim 1,wherein the material is fibrous polyester.
 3. An active field polarizedmedia air cleaner in accordance with claim 1, wherein the material isfiberglass.
 4. An active field polarized media air cleaner in accordancewith claim 1, wherein the material is a non-woven material.
 5. An activefield polarized media air cleaner in accordance with claim 1, whereinthe material is subject to UV light.
 6. An active field polarized mediaair cleaner in accordance with claim 1, wherein the material comprisesmultiple layers, wherein at least one of the layers is treated by aphotocatalyst.